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Later today, Microsoft will play it safe by patching a Windows 7 bug that it says can't be exploited.

Of the 11 security bulletins that will be released in a few hours, "Bulletin 7" will address one or more vulnerabilities in Windows 2000, Windows XP, and Windows Server 2003.

[ Windows 7 is making huge inroads into business IT. But with it comes new security threats and security methods. InfoWorld's expert contributors show you how to secure the new OS in the "Windows 7 Security Deep Dive" PDF guide. ]

But Microsoft will also offer the same update to users running Windows Vista, Windows 7, and Windows Server 2008, even though the company maintained last week that they were impervious to attack.

"Windows 7 users will be offered Bulletin 7 as a defense-in-depth update even though the [advanced notification] states that the issue does not affect Windows 7," said Jerry Bryant, a group manager with the Microsoft Security Response Center (MSRC), in one of several emails replying to questions. "This means that the vulnerable code is in the software, but due to the improved protections built into Windows 7, there are no known vectors to reach it."

In other words, the vulnerability is there -- in Vista, Windows 7, and Server 2008 -- but Microsoft doesn't know how it could be exploited.

Better safe than sorry, security experts said.

"Absolutely, it's a good practice to fix these bugs," said Andrew Storms, director of security operations at nCircle Network Security. "Just a year ago, DEP [data execution prevention] and ASLR [address space layout randomization] were mitigating nearly every vulnerability for Internet Explorer on Vista. Yet we are seeing a steady rise in more researchers' finding and taking advantage of DEP bypass methods. And if DEP bypass continues to happen more often, then we'll be happy that Microsoft issued these fixes."

Wolfgang Kandek, chief technology officer of security risk and compliance management provider Qualys, not only agreed, but cited DEP and ASLR circumventions as well. "Installing the update for Windows 7, Server 2008 and Vista is definitely a recommended, and preemptive, action," he said. "We have seen cases in the past where attackers were able to string together multiple vulnerabilities to reach their goal, most recently at CanSecWest where Peter Vreugdenhil used two bypasses to first get by ASLR, then DEP when he exploited IE8."

For his exploit expertise, Vreugdenhil, a Dutch freelance researcher, last month won $10,000 during the Pwn2Own hacking contest. His one-two punch was called "particularly impressive" by the contest organizer because it sidestepped DEP and ASLR, two cornerstones of Vista's and Windows 7's security.

Microsoft has patched untouchable vulnerabilities before, Bryant confirmed, citing several examples, including MS09-032 , a July 2009 update that disabled a company-made ActiveX control said to be unexploitable in Vista or Server 2008. Prior to that, Microsoft issued MS09-015 (April 2009) and MS08-062 (October 2008) for similar defense-in-depth reasons.

"These are typically situations where the vulnerable code exists but is not accessed in any way by the system," said Bryant. "We feel it's important to continuously look at ways to reduce the overall attack surface, so defense-in-depth updates will continue to be offered to customers."

Storms argued that it was just Microsoft following established protocol. "This probably has more to do with the mechanics of Microsoft's SDL [software development lifecycle] than anything else," Storms said. "A bug was found and validated, they are thus committed to issuing the fix. All supported platforms must receive the fix, even if it means that the possibility of an attacker even thinking about exploiting it is unlikely."

Bulletin 7 has a back story that makes it more interesting than most. Microsoft doesn't map pre-patch bulletins to issues it has said will be addressed, but clues point to Bulletin 7 fixing a flaw in VBScript. Last month, a Microsoft advisory warned Windows XP users of a bug in the scripting language, and told them not to press the F1 key when prompted by a Web site.

Storms acknowledged that the situation was probably confusing to users, what with Microsoft claiming that Vista, Windows 7 and Server 2008 were not affected by the vulnerability, but still urging users to patch. But look at the bigger picture, he said.

"[Microsoft's] basically saying that in order for an attack to work, many layers would have to be peeled apart first, essentially making an attack unlikely," he said. "Yet, Microsoft is saying they are still bound by an obligation to issue the fix."

Microsoft will release the 11 security updates, including Bulletin 7, at approximately 1 p.m. ET today.